1. Field Of The Invention
This invention relates to an apparatus and method for measuring the operating speed of devices having rotating parts, and more particularly, to acoustic tachometers for diesel engines and the like.
2. Description Of The Prior Art
Tachometers for gasoline internal combustion engines are generally connected to the electrical system of the engine. Temporary installation for engine servicing and the like is relatively simple, and may be accomplished by clipping leads to the distributor points. Since diesel engines do not have points or spark plugs, tachometers are generally connected to a moving part of the engine such as a gear, cam shaft or the like. This is often not practical for temporary installations as may be needed, for example, for setting the idle speed and governor speed, or for performing other service functions.
Optical tape may be permanently secured to the flywheel of a diesel engine for measuring engine speed with an optical tachometer. Additional mechanical connections are not needed for servicing, but the tape may be damaged fairly quickly from normal wear, and the operator must work fairly close to the engine fan to make measurements, which is sometimes dangerous.
Acoustic tachometers have been used to determine revolutions per unit time, or the operating speed, of diesel engines and other devices. Such tachometers are particularly adaptable to diesel engines where the fundamental frequency component of the sound generated by the engine combustion occurrences can be isolated from the harmonic components of the combustion occurrence sounds, and the other sounds generated by the engine, and ambient noise.
Acoustic tachometers may include a vibration pickup or other transducer affixed to a part of the engine for converting mechanical vibrations to corresponding electrical signals. In Miller U.S. Pat. No. 3,289,077 a transducer is secured to a fuel injection valve which vibrates at a frequency proportional to the speed of a diesel engine. In Takama U.S. Pat. No. 4,173,896 a sensor block is secured to a fuel line, and responds to changes in the fuel line pressure to produce signals from which engine speed may be determined. In either case, the mechanical connection requires time for installation, and readings may be inaccurate if installation is improper.
It is also known to utilize a microphone placed near a rotating device for detecting the speed of rotation of the device. Rohner U.S. Pat. No. 3,815,426 discloses a device for measuring the speed of rotation of a false-twisting machine from the center frequency of the narrow band level peak of the sonic and ultrasonic spectrum produced by the machine. A directional microphone picks up the machine sounds and converts them to corresponding electrical signals. A fixed band-pass filter with a bandwidth of 5 to 50 kHz filters these signals. The highest amplitude signals in this range are isolated by the adjustable threshold level of a Schmitt trigger circuit. The trigger circuit then produces uniform pulses at the rate of the signals at the highest amplitude. Such pulses are then counted per unit time to indicate rotational speed of the machine. One difficulty with the device is that the discrimination level is not automatically adjusted to a level just below the peak and consequently may accept any loud noises that exceed the discrimination level used, even though not at the frequency of the true peak. Furthermore, as the filter passes frequencies over a relatively wide range, the many frequencies present in the range may produce superimposed or composite signals exceeding the discrimination level even though not individually above the level. Another difficulty is that the threshold level must be recalibrated if the microphone is moved significantly during operation.
Frarey et al. U.S. Pat. No. 3,400,578 discloses an engine analyzer, as for high speed turbojet engines, in which nondirective microphones pick up radiated acoustic energy from the engine under test and produce corresponding electrical signals. The electrical signals are filtered with a band-pass filter having a center frequency corresponding to the reference speed of the engine part being tested. The filtered signal is applied to a phase lock detector having a voltage controlled oscillator (VCO) which then produces a signal at a relatively pure frequency corresponding to the frequency of the principal signal passing the filter. A control channel provides a signal dependent upon the output frequency of the VCO, which signal is applied to a balanced modulator along with the amplified output of the microphone. The modulated signal is filtered and amplified and then detected to provide an indication of the amplitude of the microphone output at the frequency of interest. A difficulty with this analyzer is that it requires a narrow band for the band-pass filter in order to reject unwanted signals, and therefore, is intolerant of such major variations in engine speed as would cause the desired signal to fall outside the pass band.
Thus, there is a need for an acoustic tachometer for diesel engines and the like which is accurate over a relatively wide range of engine speeds and yet is not mechanically connected to the engine, and may be operated at a safe distance from the engine. There is also a need for an acoustic tachometer which measures operating speed at various distances from the engine without the need for making manual adjustments.
Accordingly, one aspect of this invention is to provide a new and improved method and apparatus for measuring the operating speed of devices having rotating parts.
Another aspect is to provide a new and improved acoustic tachometer for diesel engines and the like.
Still another aspect is to provide a new and improved acoustic tachometer for diesel engines which is responsive to the fundamental frequency component of the sound generated by the engine combustion occurrences.
A still further aspect is to provide an acoustic tachometer having error detection circuitry which identifies erroneous readings caused by excessive or inadequate acoustic inputs.
Yet another aspect is to provide an acoustic tachometer which is operable over a range of distances from the engine, without making manual adjustments.
A further aspect is to provide a band-pass filter having a variable center frequency and a substantially constant bandwidth over a frequency spectrum of interest.